Wednesday, October 6, 1999 Published at 11:05 GMT 12:05 UKSci/TechEvening clouds on a Martian volcanoLooking down through the clouds at Arsia MonsBy BBC News Online Science Editor Dr David Whitehouse

As if to show that the loss of the Mars Climate Orbiter spacecraft last week
was just a relatively minor setback in the exploration of the Red Planet, the
Mars Global Surveyor (MGS) has just celebrated two years in orbit by sending
back some stunning new pictures.

MGS has imaged one of the largest volcanoes not just on Mars but anywhere in
the Solar System, Arsia Mons.

A cleft in the Tharsis plain less than one kilometres across. You can just pick out the dunes.

This so-called shield volcano is actually part of a string of three volcanoes known as the Tharsis Montes. They were called Montes or mountains before it was realised that they were extinct volcanoes.

The three volcanoes, Ascraeus Mons, Pavonis Mons and Arsia Mons, form one
of the most striking series of surface features on the planet.

These huge, broad volcanoes rise out of the Tharsis plain, spaced in a line
about 700 km (435 miles) apart. The Tharsis plain stretches for thousands of
kilometres north-west from the chaotic upland region where the mighty Valley of the Mariners can be found.

Martian floodwaters

Billions of years ago, this region of Mars must have been a magnificent and
foreboding sight. Torrential floods from the uplands created the Valley of
the Mariners and the floodwater gushed onto the plains separating into a
myriad of tributaries from which the water evaporated.

On the horizon would have been the three mighty volcanoes of Tharsis
bellowing volcanic gas into the atmosphere and drowning the region with vast
lava floes.

The Lander is due in December

On Mars, Arsia Mons is surpassed in size only by the enormous Olympus Mons, an extinct volcano which can be found a bit further to the north west.

In the new image, bright water ice clouds (the whitish-bluish wisps) can be
seen hanging above the volcano. Such clouds are a common sight nearly every
Martian afternoon in this region.

Mars Global Surveyor, launched in November 1996, has been in orbit around
Mars since September 1997, and is returning unprecedented detail about
planet's surface features, atmosphere, and magnetic properties.

With the loss of the orbiter last week, it has been decided that MGS will have to act as a part-time relay for the Mars Polar Lander, due to land on 3 December. The lander is also capable of sending data directly
to Earth.